1. Field of the Invention
The present invention relates to a device and method for measurement, especially a device and method for optical nanoindentation measurement.
2. Description of the Related Art
The wide application of thin film in the semiconductor, micro-mechanical, solar energy, and display industries over the past few years has made the mechanical properties of thin film an influencing element in deciding the performance and the service life of product. The thickness of thin film is so thin that it distinguishes itself from bulk materials in terms of mechanical properties, and so much so that traditional experimental design has been rendered inadequate to measure the mechanical properties of thin film. To tackle the problem, a variety of methods, such as bulge test, nanoindentation test, and micro tensile test, etc., have been developed in attempts to measure the mechanical properties of thin film. Nanoindentation test, in particular, has attracted strong audience in both academia and industry for its accessibility and straightforwardness. Current nanoindentation measurement systems used by academia and industry are all designed and made by international manufacturers, capable of measuring the reduced modulus and the hardness of thin film. However, the reduced modulus obtained represents only the relationship between the Young's modulus and the Poisson's ratio of the thin film, not the Young's modulus and the Poisson's ratio, respectively. Or, an estimate of Poisson's ratio has to be made before the Young's modulus can be obtained. Nevertheless, if the estimation of the Poisson's ratio is significantly different from the reality, the accuracy of the obtained Young's modulus will be compromised, too. In addition, traditional nanoindentation measurement systems are unable to measure the density, another important mechanical property, of the thin film as well.
FIG. 5 shows a schematic view of thin film 51 indentation process undertaken by traditional nanoindentation measurement system. In this figure, A-A′ represents the indentation of the thin film 51 under the applied load F of the indenter tip; B-B′ represents the residual indentation of the thin film 51 after the removal of the indenter tip; and C-C′ represents the initial surface of the thin film 51. The relationship between the maximum indentation depth hmax, the contact depth hc, and the distance between the contact position and the initial surface of the thin film 51, hs, is defined as follows: hmax=hc+hs. Besides, according to Oliver and Pharr's findings, the unloading data illustrated in FIG. 6 can be represented by a power-law function: P=K(h−hf)m, where P is the applied load, hf is the residual depth, and K and m are constants fitted by the unloading experimental data. The contact stiffness S is defined as the slop of the unloading curve at the time the maximum load is applied, as FIG. 6 shows, and can be obtained through the formula
  S  =                    ⅆ        P                    ⅆ        h              ⁢          |              h        =                  h          max                      .  Moreover, the area function A of the indentation tip of the thin film 51 can be obtained by applying the following formula:A(hc)=C0hc2+C1hc1+C2hc1/2+C3hc1/4+ . . . +C8hc1/128,where C0 to C8 are fitted constants. And, by applying the theory of contact mechanics, the following relation can be obtained:
            h      s        =          ɛ      ⁢                          ⁢                        P          max                S              ,where ε represents a constant determined by the geometry of the indenter tip. Besides, because Pmax and S are known, hs can be obtained as well. In addition, based on the formula hmax=hc+hs, hc is obtained, and, therefore, the area function A(hc) is obtained.
Finally, by applying the contact stiffness S and the area function A of the indentation tip to the following formula, the reduced modulus Er of the thin film 51 can be obtained:
      E    r    =            S      2        ⁢                  π        A            
However, due to the fact that the relationship between Er, the Young's modulus, and the Poisson's ratio is defined by
            1              E        r              =                            1          -                      v            1            2                                    E          1                    +                        1          -                      v            2            2                                    E          2                      ,traditional nanoindentation systems, as mentioned above, are only able to provide the relationship between the Young's modulus and the Poisson's ratio, not the respective values thereof. Therefore, to improve the accuracy of the Young's modulus of the thin film measured by traditional measurement systems and to enhance both the capacities and techniques of current nanoindentation measurement systems, a measuring technique combing nanoindentation and optical interference is developed, not only to improve the accuracy of the Young's modulus of the thin film measured by traditional nanoindentation measurement systems, but also to make possible the measurement of the Poisson's ratio and the density of the thin film otherwise unobtainable through traditional nanoindentation measurement systems. As a result, not only can nanoindentation test be made much more competitive in the market, but the mechanical properties of thin film can be better understood as well.